The present invention relates to the art of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with positron emission tomography (PET), and will be described with particular reference thereto. It is to be appreciated that the present invention is amenable to single photon emission computed tomography (SPECT), whole body nuclear scans, positron emission tomography (PET), Compton scattering, other diagnostic modes and/or other like applications. Those skilled in the art will also appreciate applicability of the present invention to other applications where a plurality of pulses tend to overlap, or xe2x80x9cpile-upxe2x80x9d and obscure each other.
Diagnostic nuclear imaging is used to study a radio nuclide distribution in a subject. Typically, one or more radiopharmaceutical or radioisotopes are injected into a subject. The radiopharmaceutical are commonly injected into the subject""s bloodstream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceutical. A gamma or scintillation camera detector head is placed adjacent to a surface of the subject to monitor and record emitted radiation. Often, the detector head is rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. This data is reconstructed into a three-dimensional image representative of the radiopharmaceutical distribution within the subject.
Each detector head typically includes an array of photo multiplier tubes facing a large scintillation crystal. Each received radiation event generates a corresponding flash of light that is seen by the closest photo multiplier tubes. Each photo multiplier tube that sees an event puts out a corresponding analog pulse, pulses from tubes closest to the flash being bigger than pulses from further away tubes. The analog pulses from the individual PMT""s are digitized and combined digitally to generate x and y spatial coordinates approximating the location of the scintillation event in the crystal.
As demands are made for increased patient throughput and improved image quality, the detector heads are subjected to increasing volumes of gamma ray events per second. For example, in a PET scanner in order to obtain about 150 coincident events per second, each detector head typically receives on the order of 2,000,000 events per second. Indeed, one way to increase scanning speed is to increase the number of events per second. Undesirably, as the number of events per second increase, scintillation pulse events begin to overlap to a greater and greater extent leading to pulse loss and other image degradations.
To accommodate higher count rates, current emission detector heads shorten the pulse tails produced by some of the photo multiplier tubes, reducing perceived event overlap. This analog technique is known as delay line clipping. This method desirably reduces the effect of pulse pile-up. However it tends to degrade spatial and energy resolution of a gamma camera if a commensurate shortening of integration time occurs. Moreover, scintillation pulses or events that occur without a pile-up are also subject to delay line clipping. In other words, even when the problem of pulse pile-up is not present, the signals are still clipped. This is typically seen as an engineering compromise between count rate and energy and spatial resolution. An additional technique to reduce the effects of pile-up employs the use of an analog signal extrapolation circuits to correct both the initial or first pulse, and a following pulse. Unfortunately, the number of pulses that can be corrected is limited by the number of estimator/amplifier circuits.
The present invention contemplates a new and improved method and apparatus which overcomes the above-referenced problems and others.
In accordance with one embodiment of the present invention, a nuclear camera includes detector heads mounted for movement around an examination region and a processor for reconstructing signals from the detector heads into an image representation. Each detector head includes a scintillation crystal that converts each received radiation event into a flash of light. The detector heads also include an array of photo multiplier tubes arranged to receive the light flashes and, in response, generate an analog tube output pulse. A plurality of analog-to-digital converters convert the analog tube output pulse for each photo multiplier tube to a series of digital tube output values. A processor reconstructs the image representation from the digital tube output values.
In accordance with another aspect of the present invention, a nuclear camera further includes a storage device loaded with an estimator function derived from a calibration radiation event. The processor integrates the series of digital values corresponding to each pulse until the end of the pulse or the beginning of another overlapping pulse is detected. In response to detection of the overlapping pulse, the processor accesses the estimator function in the storage device to estimate a remainder of the first pulse.
In accordance with another aspect of the present invention, the estimator function includes a plurality of ratios of portions of the calibration event at selected sampling times. The processor includes a multiplier for multiplying a ratio corresponding to a sampling characteristic of the first event at the time which the second event is detected and the integrated digital values between detection of the first and the second events. This produces an estimated remainder. An adder is also provided for adding the integrated digital values and the estimated remainder together to arrive at an estimate of the first event.
In accordance with another aspect of the present invention, the estimator function includes a plurality of estimated remainders corresponding to integrated digital values and sampling times. The processor adds the integrated digital values and the corresponding estimated remainder to provide an estimate of the first scintillation event.
In accordance with another embodiment of the present invention, in a medical imaging device for imaging an area of interest by detecting radiation events, a method of estimating truncated events includes determining at least one estimator function for the medical imaging device by sampling one calibration event at least. The method further includes detecting a first event of interest followed by detection of a second event interrupting the first event of interest. Based on the estimator function, a remainder of the first event of interest is estimated following detection of the interrupting second event.
In accordance with another aspect of the present invention, the method further includes digitally sampling the detected first event at a predetermined sampling rate while combining the digital samples. Upon detection of the second event, the combining of samples is ceased.
In accordance with another aspect of the present invention, the method further includes totaling the combined digital samples of the detected event and the estimated remainder resulting in an estimate of the entire first event.
In accordance with another aspect of the present invention, the determining an estimator function includes calculating ratios of combined digital samples at selected sampling times, to a sum of digital samples combined over the entire calibration event.
In accordance with another aspect of the present invention, the estimating a remainder step includes based on a sampling time substantially coincident with the detection of the interruption, retrieving a calculated ratio. The combined digital sample is then multiplied with the retrieved ratio resulting in an estimate of the remainder of the first event.
In accordance with another aspect of the present invention, the determining an estimator function includes associating combined digital samples from the calibration event with the calculated ratios to produce a plurality of remaining area estimates.
In accordance with another aspect of the present invention, the estimating a remainder step includes determining a one of the remaining area estimates corresponding to combined digital samples and a sample time of detection of the second event.
In accordance with another aspect of the present invention, upon detection of the second event, samples of the coincident events are combined, and the estimated remainder of the first event is removed from the combination to yield an estimate of the second event.
In accordance with another embodiment of the present invention, a method of estimating an energy of an event detected by a medical imaging device includes detecting a first event. The detected event is sampled at a defined sampling rate while samples from the detected event are combined. When a second event is detected, partially coincident with the first event, the combining is ceased. A remainder representing an estimate of combined samples of the first event is estimated following detection of the second event.
In accordance with another aspect of the present invention, the method further includes for the first event, totaling the combined samples of the detected first event and the estimated remainder of the first event thus producing an estimate of the first event.
In accordance with another aspect of the present invention, the method further includes on detection of the second event, combining sampled values to generate a second event combined value. The estimated remainder of the first event is removed from the second combined value, yielding an estimate of the second event.
In accordance with another aspect of the present invention, the method further includes determining a family of estimator functions for the medical imaging device from calibration events.
In accordance with another aspect of the present invention, the estimator function determining step includes calculating ratios of combined samples up to selected sampling times, to combined samples over the entire calibration event.
In accordance with another aspect of the present invention, the remainder estimating step includes, based on a sampling time substantially coincident with the detection of the second event, retrieving a corresponding ratio. The combined samples of the first event are then multiplied with the retrieved ratio, yielding an estimate of the remainder.
In accordance with another aspect of the present invention, the estimator function determining step further includes combining calibration samples up to selected times with the corresponding ratios thereby producing a plurality of remaining area estimates.
In accordance with another aspect of the present invention, the remainder estimating step includes retrieving a remaining area estimate based on combined samples of the first event before detection of the second event, and the detection time of the second event.
In accordance with another aspect of the present invention, the remainder estimating step includes determining an area under at least two points of an estimated pulse tail curve generated from the calibration events.
One advantage of the present invention resides in improved energy and spatial resolution.
Another advantage is that pulse tail estimation is not limited to fixed integration times. The pulse can be integrated to the maximum integration time when pulse pile-up does not occur.
Another advantage of the present invention resides in the elimination of extra extrapolation/amplifier circuits. The correction for pulse pile-up can be completed at the sampling rate of the digital sampling circuitry without extra circuits to handle multiple pile-up events.
Yet another advantage of the present invention resides in a calibration procedure using a statistically significant number of pulses to determine the estimator function. The estimator only assumes that the pulse shape is consistent for the correction to be accurate. In other words, no knowledge of the pulse shape is required. Different estimator coefficients can be selected for a given channel""s optics or electronics or operating environment, such as temperature.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.